1. Field of the Invention
This invention relates, in general, to the determination of engine mass air flow and, in particular, to a method and apparatus for determining the mass air flow using stochastic estimation techniques, thereby eliminating the need for a mass air flow sensor.
2. Description of the Art
The control system of an internal combustion engine usually incorporates both a manifold air pressure (MAP) sensor and a mass air flow (MAF) sensor. These sensors serve as inputs to the engine controller to control the emission and performance characteristics of the engine. In particular, knowing the mass of air available for combustion within engine cylinders allows adjustment of critical engine parameters, particularly engine fueling.
Mass air flow sensors are commercially available and have been used with internal combustion engines in the past to provide the required information regarding the air mass available for combustion. At the present time, however, MAF sensors are relatively expensive as compared to other sensors used for engine control. Therefore, alternative techniques of deriving mass air flow have developed.
Two conventional techniques for calculating mass air flow without the use of MAF sensor exist. The speed density method of calculating mass air flow generally incorporates the output of the MAP sensor, a measure of the engine""s speed, and intake air temperature. The throttle position method of calculating mass air flow generally incorporates the output of the throttle position sensor and engine speed, among other variables. While the speed density and throttle position methods of calculating mass air flow eliminate the expensive MAF sensor, they are also more inaccurate than using the MAP sensor. First, they may provide an incorrect estimation of air flow during throttle transient conditions because in the finite amount of time it takes to calculate air mass and adjust fuel input, the air flow can change dramatically due to the dynamic nature of an engine. Even during static situations, the use of these methods results in cycle-to-cycle measurement variations (noise) due to the pulsations caused in air flow as the engine draws air into its different cylinders and due to delays in processing sensor information. This noise has been shown to be detrimental to emissions in that a 1% noise increase in air flow measurement can result in a 15% HC increase.
Because of the increase in inaccuracies over the use of a MAF sensor, these traditional methods can result in a need for more expensive emission system hardware over that needed when the MAF sensor is used.
Thus, there is a need to determine the mass of air available for combustion within the cylinders of an internal combustion engine without the use of a MAF sensor. Further, the technique used should produce estimations of mass air flow at least as accurate as the MAF sensor.
The present invention is directed toward providing a method and apparatus for determining the mass of air available for combustion within the cylinders of an internal combustion engine, with sufficient accuracy to enable proper engine control, without requiring a mass air flow (MAF) sensor. The mass of air available for combustion is obtained by combining a throttle position mass air flow estimate with a speed density mass air flow estimate through a first Kalman filter.
According to one aspect of the invention, the input into the speed density mass air flow estimate is the output of a manifold air pressure (MAP) sensor, certain engine operating conditions, and a calibratable estimation of volumetric efficiency. The input into the throttle position mass air flow estimate is throttle position and certain engine operating conditions, together with compensation for the minimum mass air flow variation.
In a preferred aspect of the invention, the throttle position mass air flow estimate uses an estimated throttle position obtained from a second Kalman filter as its input throttle position. This throttle position Kalman filter has as its input the desired throttle position, provided by a pedal position sensor, and the measured throttle position, provided by the throttle position sensor. According to another aspect of the invention, the throttle position mass air flow estimate uses the measured throttle position from the throttle position sensor as its input throttle position.
In one aspect of the invention, once the mass air flow is determined as described above, it is reported to the engine controller for use in controlling certain engine parameters, particularly the air-to-fuel ratio.
The present invention eliminates the need for the expensive mass air flow sensor through the unique combination of a speed density mass air flow estimate and a throttle position mass air flow estimate using Kalman filtering techniques. The present invention is intended to reduce the cycle-to-cycle noise resulting from the use of either traditional method. Further, the use of Kalman filtering in mass air flow estimation is intended to reduce errors in air flow estimation resulting from throttle transient conditions. The present invention is thus intended to at least achieve the accuracy of the mass air flow sensor, potentially eliminating the need for a more expensive emission system than that needed when either traditional method is used alone.